It has heretofore been known that .alpha.-olefin polymers, in particular, ethylene polymers, or ethylene .alpha.-olefin copolymers are prepared by processes wherein ethylene is polymerized or ethylene and .alpha.-olefin are copolymerized in the presence of a titanium based catalyst composed of a titanium compound and an organoaluminum compound, or a vanadium based catalyst composed of a vanadium compound and an organoaluminum compound.
Generally, ethylene/.alpha.-olefin copolymers obtained with the titanium based catalysts have a broad molecular weight distribution and are poor in transparency, surface non-tackiness and dynamical properties. Ethylene/.alpha.-olefin copolymers obtained with the vanadium based catalysts have a narrower molecular weight distribution and a narrower composition distribution in comparison with the ethylene/.alpha.-olefin copolymers obtained with the titanium based catalysts, and show a fairly good improvement in transparency, surface non-tackiness and dynamical properties, but said ethylene/.alpha.-olefin copolymers are found to be insufficient for practical purposes wherein such properties are required. Therefore, it is desired that ethylene/.alpha.-olefin copolymers further improved in these properties are developed to make their appearance.
Under such circumstances, on the one hand, there have been proposed recently processes for the preparation of ethylene/.alpha.-olefin copolymers using catalysts composed of zirconium compounds and aluminoxanes as a new type of Ziegler catalysts for olefin polymerization.
For example, Japanese L-O-P Publn. No. 19309/1983 discloses a process for the preparation of ethylene/.alpha.-olefin copolymers, which process comprises polymerizing ethylene and one or at leat two C.sub.3 -C.sub.12 .alpha.olefins at a temperature of from -50.degree. C. to 200.degree. C. in the presence of a catalyst composed of a transition metal-containing compound represented by the following formula EQU (Cyclopentadienyl).sub.2 MeRHal
wherein R is cyclopentadienyl, C.sub.1 -C.sub.6 alkyl or halogen, Me is a transition metal, and Hal is halogen, and a linear aluminoxane represented by the following formula EQU Al.sub.2 OR.sub.4 (Al(R)--O).sub.n
wherein R is methyl or ethyl, and n is a number of 4 to 20, or a cyclic aluminoxane represented by the following formula ##STR1## wherein R and n are as defined above. This patent L-O-P publication teaches that in order to regulate the density of the resulting polyethylene, the polymerization of ethylene should be carried out in the presence of small quantities of up to 10% by weight of fairly long chain .alpha.-olefins or mixtures thereof.
Japanese Patent L-O-P Publn. No. 95292/1984 discloses an invention relating to processes for preparing a linear aluminoxane represented by the following formula ##STR2## wherein n is a number of from 2 to 40, and R is C.sub.1 -C.sub.6 alkyl, and a cyclic aluminoxane represented by the following formula ##STR3## wherein n and R are as defined above.
This patent L-O-P publication describes that when the polymerization of ethylene is carried out in the presence of a mixture comprising the aluminoxane prepared by the process of said publication, for example, methyl aluminoxane, and a bis(cyclopentadienyl)zirconium compound or a bis(cyclopentadienyl)titanium compound) there are obtained at least twenty-five million grams of polyethylene per 1 g of the transition metal and per 1 hour.
Japanese Patent L-O-P Publn. No. 35005/1985 discloses a process for preparing olefin polymerization catalyst, which comprises first reacting an aluminoxane compound represented by the following formula ##STR4## wherein R.sup.1 is C.sub.1-C.sub.10 alkyl, and R.sup.0 is R.sup.1, or R.sup.0 represents --O-- by linkage, with a magnesium compound, and then chlorinating the reaction product, followed by treatment with a compound of Ti, V, Zr or Cr. This patent L-O-P publication describes that the catalysts prepared by the process are particularly suitable for use in copolymerization of ethylene and C.sub.3 -C.sub.12 .alpha.-olefins.
Japanese Patent L-O-P Publn. No. 35006/1985 discloses a combination of (a) two or more dissimilar mono-, di- or tri-cyclopentadienyls or their derivatives of transition metals and (b) an aluminoxane as a catalyst for preparation of reactor blended polymers. Example 1 of this patent L-O-P publication discloses that ethylene and propylene are polymerlzed in the presence of a catalyst composed of bis(pentamethylcyclopentadienyl)dimethyl zirconium and an aluminoxane to obtain polyethylene having a number average molecular weight of 15,300, a weight average molecular weight of 36,400 and containing 3.4% of the propylene component. In Example 2 of this patent L-O-P publication, ethylene and propylene are polymerized in the presence of a catalyst composed of bis(pentamethylcyclopentadienyl)zirconium dichloride, bis(methylcyclopentadienyl)zirconium dichloride and an aluminoxane to obtain a blend of polyethylene and an ethylene/propylene copolymer consisting of a toluene-soluble portion having a number average molecular weight of 2,200 and a weight average molecular weight of 11,900 and containing 30 mol % of the propylene component and a toluene-insoluble portion having a number average molecular weight of 3,000 and a weight average molecular weight of 7,400 and containing 4.8 mol % of the propylene component, said blend having a number average molecular weight of 2,000 and a weight average molecular weight of 8,300 and containing 7.1 mol % of the propylene component. Similarly, Example 3 discloses a blend of LLDPE and an ethylene/propylene copolymer consisting of a soluble portion having a molecular weight distribution (Mw/Mn) of 4.57 and containing 20.6 mol % of the propylene component and an insoluble portion having a molecular weight distribution of 3.04 and containing 2.9 mol % of the propylene component.
Japanese Patent L-O-P Publn. No. 35007/1985 discloses a process wherein ethylene alone is polymerized, or ethylene and an .alpha.-olefin of 3 or more carbon atoms are copolymerized in the presence of a catalyst system containing a metallocene and a cyclic aluminoxane represented by the following formula ##STR5## wherein R is an alkyl group of 1 to 5 carbon atoms, and n is an integer of 1 to about 20, or a linear aluminoxane represented by the following formula ##STR6## wherein R and n are as defined above.
According to this patent L-O-P publication, the polymers obtained in this manner have a weight average molecular weight of about 500 to about 1,400,000 and a molecular weight distribution of 1.5 to 4.0.
Japanese Patent L-O-P Publn. No. 35008/1985 discloses that polyethylene or ethylene/C.sub.3 -C.sub.10 copolymers having a wide molecular weight distribution are prepared by using a catalyst system comprising at least two metallocenes and aluminoxane. This patent L-O-P publication indicates that the copolymers obtained in this manner have a molecular weight distribution (Mw/Mn) of 2-50.
Japanese Patent L-O-P Publns. Nos. 260602/1985 and 130804/1985 propose processes for polymerizing olefins by using catalysts formed from a mixed organoaluminum compound comprising a transition metal compound, aluminoxane and an organoaluminum compound, and disclose that polymerization activity per unit transition metal improves by the addition of the organoaluminum compound.
In every process cited hereinbefore, however, there was involved such a problem that activity per unit aluminoxane of the catalyst used is still low. Further, there was such a problem that it is difficult to obtain polymers having a sufficiently high molecular weight when olefins, for example, ethylene and propylene are copolymerized by using catalysts formed from the transition metal compounds and aluminoxanes hitherto known.